1. Field of the Invention
The invention relates to a method of manufacturing multilayer mirrors, in which a first layer of a first material and of a first thickness is deposited on a substrate, a second layer of a second material and of a second thickness being deposited on the first layer, and material being removed from the second layer by ion-etching the material by ion bombardment, the foregoing steps being repeated at least once.
The invention also relates to a multilayer mirror manufactured by means of the method of the invention.
2. Description of the Related Art
A method of this kind is disclosed in an article "Ion etching of thin W-layers: enhanced reflectivity of W-C multilayer coatings", Applied Surface Science 47 (1991), (Elsevier Science Publishers, North-Holland), pp. 63 to 76.
The term "multilayer mirror" is to be understood to mean not only a multilayer mirror which is used in the visible part of the spectrum of electromagnetic radiation, but also in other (notably shortwave) parts thereof, or for reflection of elementary particles such as neutrons.
Known multilayer mirrors are manufactured by providing materials with two different refractive indices on one another by physical deposition. The reflectivity of one interface between such layers for electromagnetic radiation in a wavelength range smaller than, for example, 50 nm amounts to some tenths of a percent for angles larger than the critical angle. Reflection of electromagnetic radiation by a multilayer mirror is based on interference between radiation reflected at the many interfaces of the multilayer mirror, and is suitably approximated by the known Bragg relation. Therefore, this reflection is of a dispersive nature. For reflection angles larger than the critical angle, reflection amounting to some tens of percents can thus be obtained. Multilayer mirrors are thus used to provide high reflectivity at large angles relative to the surface of the layers and can also be used as dispersive elements because of their dispersive nature. A multilayer mirror for the reflection of shortwave radiation consists of successive periods of each time two layers of materials of a different refractive index and of a thickness of the order of magnitude of the wavelength of the reflected radiation. For shortwave radiation a difference in refractive index can be considered as a difference in density, i.e. the atomic number.
The total reflectivity of a multilayer mirror is determined by the magnitude of the reflection per interface, i.e. by the difference in refractive index. A large difference in refractive index implies that one of the materials should have a high density; it is an inevitable consequence thereof that the relevant layer has a low transparency for the reflected radiation. This means that only a limited number of bilayer periods can be used to contribute to the total reflected radiation.
Because the reflection, as has already been stated, is of a dispersive nature, these multilayer mirrors can also be used as dispersive elements. The wavelength resolution is proportional to the number of bilayer periods in the multilayer mirror participating in the total reflection. Absorption of electromagnetic radiation in the layers of high density thus imposes a limit as regards the wavelength resolution.
Another problem encountered in the manufacture of multilayer mirrors resides in the roughness of the interfaces between the layers. For optimum reflectivity of a multilayer system, the roughness of the interfaces should not be greater than the dimensions of the atoms of the deposited material (.apprxeq.0.2 nm). Roughness is more important as the bilayer period is smaller. Roughness of an interface is introduced in the form of surface roughness during the deposition process of the materials. When a next layer is deposited, the roughness of the surface of this layer may increase. For layer thicknesses of less than 3 nm the roughness may take the form of islands, so that not even a closed layer is grown. For systems with a small period or systems with a larger period whose metal layer thickness must be very small for reasons of absorption (i.e. &lt;2 nm), it is very likely not only that roughness of the successive interfaces occurs, but also that the formation of islands inhibits the formation of a closed layer.
The cited article describes the manufacture of multilayer mirrors in which the first one of the successive layers consists of carbon and the second layer consists of tungsten. These layers are provided one on the other by electron beam vapour deposition. The layer thickness of each of the two layers is of the order of magnitude of 1.5 nm. The cited article describes that the surface roughness of the layers can be counteracted by ion etching of the second layer. The bilayer period is adjusted to the correct thickness and surface roughnesses are removed by the growth of additional tungsten, followed by the removal by etching of this additional layer with 200 eV argon ions. It has been demonstrated that the surface of the top layer is smoothed by ion bombardment, but also that mixing of the layers around the interface may occur. This problem is described, for example in an article "Limits to Ion Beam Etching of Mo/Si multilayer coatings", Proceedings of "Physics of X-ray Multilayer Structures", Technical Digest Services, Vol. 7. Briefly speaking, this problem consists in that the ion bombardment drives a part of the material of the upper layer, exposed to the etching operation, into the layer situated therebelow. This gives rise to an undesirable situation which is comparable to severe interface roughness.